1. Field of the Invention
The present invention relates to a composite optical element including a plurality of optical elements in which at least one optical element is an element modifying an optical path.
Further, the present invention relates to a projection optical device that includes the above-described composite optical element, and projects an image on a screen.
2. Description of the Related Art
US Patent Application Publication No 20050063196 A1 discloses an optical system including a combination of a plurality of optical elements such as a light pipe constituting a light guiding path and a prism bending an optical path, for example. The light pipe represents the light guiding path that can propagate light while reducing optical loss and retaining uniformly brightness inside the pipe. In the case where the light guiding path is extended for a long distance, light is propagated using a medium of a coaxial structure such as an optical fiber having a core and clad, refractive indices of which are different. Further, it has been known that a minimum bending radius is determined by a difference between refractive indices of the core and clad, and a core diameter in such optical fiber. Accordingly, there is no need to combine a plurality of optical elements in an optical system such as a projector, if a light pipe having similar flexibility to the optical fiber is used. However, it is difficult at present to manufacture a small and reliable light guiding path using a mass-producible material having such flexibility and transparency, in other words, using a material suitable for the light pipe.
Light pipes typically use a prism to change a direction of propagating light. FIGS. 1 and 2 illustrate such optical systems respectively including a combination of a plurality of optical elements, for example. An optical system 1 shown in FIG. 1 is configured to have a combination of a rectangular prism 2 to orthogonally bend an optical path, and two light pipes 3 and 4 with the rectangular prism 2 provided in between. The rectangular prism 2 orthogonally bends incident light L entered from the light pipe 3 on one side, and guides the light L to the light pipe 4 on the other side. An optical system 6 shown in FIG. 2 is configured to have a combination of a non-rectangular prism 7 to bend the optical path in a required non-orthogonal direction, and two light pipes 8 and 9 with the non-rectangular prism 7 provided in between. The non-rectangular prism 7 bends incident light L entered from the light pipe 8 on one side at a required non-orthogonal angle, and guides the light L to the light pipe 9 on the other side.
The optical system 1 shown in FIG. 1 is configured such that air gaps (air layers) 11 are provided between the two light pipes 3, 4, and rectangular prism 2. The incident light L entered from the light pipe 3 on one side, more specifically, both of light beams La going straight and light beams Lb being guided obliquely, are totally reflected by the rectangular prism 2, and directed to the light pipe 4 on the other side (refer to FIG. 3). The optical system 6 shown in FIG. 2 is similarly configured such that air gaps 12 are provided between the two light pipes 8, 9, and non-rectangular prism 7. The incident light L entered from the light pipe 8 on one side is totally reflected by the non-rectangular prism 7, and directed to the light pipe 9 on the other side.
The reasons why those optical systems 1 and 6 are configured to have such air gaps 11 and 12 are described below. FIG. 4 shows, for example, an optical element 1′ that represents an optical system in which the rectangular prism 2 and two light pipes 3 and 4 are simply stuck to be integrated, that is, to be one component. In such optical system 1′, as shown in the figure, there is the case where the light beams Lb guided obliquely to the light pipe 3 pass through the light pipe 4 directly without being incident on a prism surface, and leaks to the outside, or the light beams Lb are reflected on the prism surface at 45°, and leaks to the outside. Therefore, the light beams Lb may not be appropriately guided to the light pipe 4 on the other side. This phenomenon is similar in the case where the optical system 6 shown in FIG. 2 is made into one integrated body by simply bonding the light pipes and prism together to be an optical element of one component.
There has been known a projection optical device (optical projector) in which an image-forming light valve such as a liquid crystal panel and a DMD (Digital Micromirror Device) is illuminated using an illumination optical system, and transmitted light or reflected light from the image-forming light valve is projected on a screen by a projection lens. In a projection optical device in general, light emitted from a light source is separated into red, green and blue each having a corresponding band of wavelength, modulated by the image-forming light valve in accordance with image information, and again combined to be projected on the screen, thereby displaying a color image.
US Patent Application Publication No. 20050063196 A1 and European Patent Application Publication No. 1581010 A1 disclose a projection optical device using a light pipe, thereby reducing the size of the device.
FIG. 5 shows an example of a projection optical device (optical projector) of the related art. A projection optical device 21 is configured to have a white light source 22, image-forming light valves 23R, 23G, and 23B corresponding to red, green and blue, illumination optical system 24 that separates light emitted from the white light source 22 into bands of wavelengths of red, green and blue so that light of respective bands of wavelengths enters the corresponding image-forming light valves 23R, 23G, 23B. Further, the projection optical device includes a cross prism 25 that is an optical element to combine color light respectively modulated in accordance with image information using the image-forming light valves 23R, 23G and 23B, and projection lens 26.
The illumination optical system 24 includes: a tapered light pipe 27 to guide the light emitted from the white light source 22, polarization beam splitter 28 to divide the light emitted from the white light source 22 into a P-wave and an S-wave, ½ wavelength plate 29 disposed at one exit surface of the polarization beam splitter 28, and first rectangular prism 30 being disposed at the other exit surface of the polarization beam splitter 28 and orthogonally bending an optical path. Further, the illumination optical system 24 includes a first dichroic prism 31 that is an optical element to divide light and is disposed at exit surfaces of the ½ wavelength plate 29 and first rectangular prism 30, second rectangular prism 33 disposed at one exit surface of the first dichroic prism 31 with a light pipe 32 in between, and second dichroic prism 35 disposed at the other exit surface of the first dichroic prism 31 with a light pipe 34 in between. Moreover, the illumination optical system 24 includes a third rectangular prism 37 disposed at one exit surface of the second dichroic prism 35 with a light pipe 36 in between, and fourth rectangular prism 39 disposed at an exit surface of the third rectangular prism 37 with a light pipe 38 in between.
The first dichroic prism 31 transmits the first color light of red, for example, and reflects the second color light and third color light of, for example, blue and green. The second dichroic prism 35 transmits the second color light of blue, for example, and reflects the third color light of green. An exit surface of the second rectangular prism 33 faces the red image-forming light valve 23R, for example. The other exit surface of the second dichroic prism 35 faces the green image-forming light valve 23G, for example. An exit surface of the fourth rectangular prism 39 faces the blue image-forming light valve 23B, for example.
The image-forming light valves 23R, 23G and 23B are formed of liquid crystal panels and polarization plates, for example. The beam splitter 28 transmits the P-wave and reflects the S-wave, for example.
Further, air gaps A, B, C, D, E1, E2, F1, F2, F3, G, H and I are formed respectively between elements modifying the optical path, and optical elements on the incident side and/or exit side of the elements modifying the optical path. The elements modifying the optical path include the polarization beam splitter 28, first and second dichroic prisms 31 and 35, first, second, third, fourth rectangular prisms 30, 33, 37, 39, and cross prism 25.
In the projection optical device 21, light emitted from the white light source 22 is directed to the tapered light pipe 27 and incident on the polarization beam splitter 28. The P-wave that is one divided component of the light incident on the polarization beam splitter 28 is transmitted through the polarization beam splitter 28, and converted into the S-wave by the ½ wavelength plate 29 to be incident on the first dichroic prism 31. On the other hand, the S-wave that is the other component of the light divided (reflected) by the polarization beam splitter 28 is orthogonally bent by the first rectangular prism 30, and incident on the first dichroic prism 31.
The light incident on the first dichroic prism 31 is divided. Red light divided is transmitted through the dichroic prism 31, and passes through the light pipe 32 and second rectangular prism 33 to be incident on the red image-forming light valve 23R. On the other hand, green light and blue light divided are reflected by the dichroic prism 31, are directed to the light pipe 34, and enter the second dichroic prism 35 to be divided. Specifically, the green light divided is reflected by the dichroic prism 35 to be incident on the green image-forming light valve 23G. The blue light divided is transmitted through the dichroic prism 35, passes through the light pipe 36, third rectangular prism 37, light pipe 38 and fourth rectangular prism 39 to be incident on the blue image-forming light valve 23B.
Color light components of red, green, and blue modulated in respective image-forming light valves 23R, 23G and 23B in accordance with image information enter the cross prism 25 and are combined. The combined image information light is magnified by the projection lens 26 and projected on the screen.